Well heck! Probably a bacterial infection. Could be it scraped on a rock while so skinny the bones protruding a bit. Bacterial infections can go sideways real fast, especially if the fish's immune system has been compromised. More from Humblefish's treatment guide:
Antibiotics: Treats bacterial infections, which are oftentimes “secondary” to preexisting parasitic infestations.
How To Treat - Follow the directions on the label of whatever product you are using. Always use a broad spectrum antibiotic medication i.e. one that treats both gram-positive and gram-negative bacterial diseases (very important). Examples of this include Furan-2, Kanamycin aka Seachem Kanaplex, Nitrofuracin Green Powder, Triple Sulfa Powder or a combination of using both Erythromycin & Minocycline. The latter can be accomplished (albeit expensively) by using freshwater Maracyn 1 & 2 and then doubling the dosage for saltwater use. When battling a particularly nasty bacterial infection, combining Furan-2, Kanaplex and metronidazole (ex. Seachem MetroPlex) can be very effective (and safe). Props to “hedgedrew” for enlightening me of this.
Normally you have to do 4 doses to complete a full course of treatment. The instructions may say to dose every 24 hours, and perform water changes at certain intervals. In my experience, you often have to do 2 full courses of treatment (or 8 doses total) before a fish will show signs of improvement (antibiotics are notoriously slow acting with fish). If a fish shows no signs of improvement after the second full course, then it’s time to try a completely different medication. Just like with humans, sometimes a certain fish will respond better to a different antibiotic. You can run activated carbon at any time when you wish to end treatment.
Pros - One of the few medications that can safely be mixed with others. You can use antibiotics in conjunction with just about everything.
Cons/Side Effects - Appetite suppression, depletes the water of oxygen (so provide additional gas exchange).
